OFDM represents a developing area of interest in both wired and wireless communication systems. OFDM simultaneously offers the promise of higher aggregate data rates, as well as more robust performance in selective fading environments.
In simple terms, an OFDM signal comprises a plurality of sub carriers that are spaced apart over a given frequency band. Thus, transmission with OFDM involves transmitting data symbols on multiple frequency sub carriers, where each sub carrier comprises a narrow band signal. Non-limiting examples of the application of OFDM to communication systems include various standards promulgated by the Institute of Electrical and Electronics Engineers, Inc. (IEEE), such as the IEEE 802.16 and 802.20 standards.
As with virtually all higher-rate communication systems having transmission signals subject to propagation channel distortion, estimation of the propagation channel in OFDM systems represents a critical element of receiver performance. To that end, OFDM transmitters generally adopt one of two conventional approaches to supporting receiver channel estimation.
In a first approach, an OFDM transmitter uses the same sub carriers to send data symbols at certain times, and to send pilot symbols at other times. Doing so allows a corresponding receiver to estimate the sub carrier channels based on receiving known pilots on those sub carrier frequencies. Of course, the duration between pilot transmissions must be short enough that the channel estimates remain valid for data symbols received between pilot transmissions.
In a second approach, the transmitter designates a certain number of the sub carriers as “pilot” sub carriers, meaning that these sub carriers exclusively carry pilot symbols. Remaining ones of the sub carriers comprising the OFDM signal carry the data symbols. In a conventional implementation of this approach, the pilot sub carriers are uniformly distributed among the data sub carriers.
Uniform distribution of the pilot sub carriers in this latter approach simplifies Maximum Likelihood (ML) estimation of the channel, but such estimation suffers from so-called “boundary” effects within the edge regions of the OFDM signal. Other, more computationally complex channel estimation algorithms can be used, such as a Minimum Mean Square Error (MMSE) estimation algorithm, but such algorithms often are prohibitively complex because of the matrix sizes involved.